As computer network systems are widely furnished in the current medical field, image output devices are handled like many other precision machines. One concern about prior art laser image setters and laser imagers is the risk of deteriorating various precision machines because potentially corrosive developer and fixer liquids are used. It is also a problem that waste liquid from such processing solution is ecologically detrimental and that processing is impossible under water-shortage situations as in disaster. There is a strong desire to develop an image forming system dispensing with processing solution. Needed in this regard is a technology relating to photothermographic imaging materials capable of forming clear black images at a high resolution and sharpness and finding use in medical diagnosis and photographic applications and thermographic imaging materials capable of forming images using thermal heads. These thermal imaging materials can offer to the customer a simple image forming system which eliminates a need for solution type chemical agents and is not detrimental to the environment.
While a similar requirement is sometimes raised in the general imaging material field, medical images require high definition, that is, high image quality characterized by excellent sharpness and graininess. From the standpoint of ease of diagnosis, blue black tone images are favorable. At present, various hard copy systems using pigments and dyes such as ink jet printers and electrophotographic printers are widely marketed as the image forming system, but none of them are satisfactory as the output system for medical images.
Thermal image forming systems utilizing organic silver salts are disclosed in the literature, for example, U.S. Pat. No. 2,910,377 and JP-B 4924/1968. Although these thermal image forming systems utilizing organic silver salts can produce images of quality and tone regarded satisfactory as medical images, the image forming ability of such material can be altered prior to image formation. Once images are formed, the images can be degraded by light, heat, and water.
It is a serious problem that photographic properties become worse, especially fog increases, during shelf storage. One common practice is to use mercury compounds for suppressing fog. The use of mercury compounds, however, is hazardous. As a substitute, various oxidizing agents, especially polyhalogenated compounds have been proposed. Materials using such oxidizing agents still suffer from the problem that discoloration occurs in images during storage.